The use of carbon dioxide for production of oil and gas from hydrocarbon containing reservoirs is well known. Utilization of liquid carbon dioxide (LCO2) in fracture treatment of oil and gas formations has certain advantages in water sensitive and low pressure formations. First, the use of LCO2 enables a significant reduction in water volume utilized, which minimizes formation damage caused by the water and second, it promotes water flow-back (i.e., retrieval of water introduced, or produced, in the fracture treatment) through expansion when pressure is let off the fractured formation. LCO2 used in fracturing treatments is typically added to a high pressure stream of water and proppant (usually sand) at the well-head. This is due, in part, because it is simpler to add proppant to water at atmospheric pressure than it is to add proppant to LCO2 at elevated pressure (i.e. greater than the triple point pressure of carbon dioxide, which is 75.1 psia).
Hydraulic fracturing is the term used to describe a process whereby a fluid is pumped into a well bore communicating with a subterranean reservoir under sufficient pressure to fracture the matrix of the subterranean geological formation. As these pressure forces increase, they commence and propagate fractures (fissures or cracks) in the reservoir matrix. The dimensions of the fractures generally increase by continuing to pump the pressurized fluid into the formation through the well bore.
An acceptable fracturing fluid must have several characteristics. Among these are the following: (1) the viscosity should be low enough to easily pump the fluid with conventional surface equipment; (2) the fluid must be viscous enough to move the proppant in suspension during the pumping operations and deposit the proppant in the fractures created in the formation; (3) the fluid must flow into the fractures created in the formation with a minimum of fluid loss to the pores within the matrix formation; and (4) the fluid must not plug the pores of the formation permanently, to ensure the production capacity of the formation for obtaining the desired oil and gas (hydrocarbons). It is generally the case that more viscous fracturing fluids will cause shorter but wider fractures. Often it is desirable to produce shorter fractures in order to keep the fractures within a desired zone of the formation, and wider fractures are of use in oil-producing formations to enable the oil to be produced and flow more readily. More viscous fracturing fluids are also known to reduce fluid loss/formation leak-off, which detracts from the efficiency of the fracturing operation.
Historically, the base fluid of many fracturing fluids (sometimes referred to as carrier fluid) has been comprised of either an aqueous fluid or a hydrocarbon fluid. Some of these base fluids can be utilized in conjunction with thickening agents (gels). Under some circumstances, for instance in slickwater fracturing—where a friction reducer allows pumping the fluid at high velocities—facilitating proppant transport, the addition of these thickening agents is not required. However, it is often the case that the base fracturing fluids are too low in viscosity to adequately maintain the proppant in suspension at normal pumping rates, therefore the addition of a thickening agent is desired.
It is normal to use a fracturing fluid without proppant to cause the initial fracture, in a pad stage, and it is possible to eliminate use of a proppant, for instance in shallow formations where fractures can remain open by themselves after fracture treatment. Normally, however, proppant is added to prop the fractures open and facilitate oil and gas recovery from the well. In addition to keeping the propping material or proppant in suspension while being pumped down the well, the fracturing fluid must also properly deposit the proppant in the fractures of the formation. In general, the higher the viscosity of the fracturing fluid, the more suitable the fluid is for purposes of proppant suspension. The higher viscosity fracturing fluids tend to hold the proppant in suspension as the fracturing fluid is pumped into the well and prevents the proppant from settling into the bottom portion of the resulting fracture. Moreover, the higher viscosity fracturing fluids tend to prevent the proppant from bridging across the fracture. If bridging of the proppant can be avoided, or the proppant does not settle at the bottom of the fracture, a longer propped fracture is obtainable and a better hydrocarbon recovery will result.
U.S. Pat. No. 4,567,947 to Canadian Fracmaster Ltd., discloses a fracturing fluid composition including at least one substantially anhydrous aliphatic alcohol, a non-ionic homopolymer to form a gel with the alcohol and as gel activating agent an alkali metal halide or an alkaline earth metal halide. Similarly, U.S. Pat. No. 4,701,270 to Canadian Fracmaster Ltd., discloses a fracturing fluid including liquid carbon dioxide which has been thickened by the addition of a small amount of a copolymer which is the reaction product of liquid carbon dioxide and an alkene oxide, preferably propylene oxide. These chemically based gel thickeners relied upon the need to be soluble in the carbon dioxide, whereas the fumed silica of the present invention do not have this constraint.
Great Britain Patent Number 1439735 to Texaco is directed to a hydraulic fracturing method for subterranean formations and describes a method for increasing the productivity of the formation by using a fracturing fluid wherein the fracturing fluid is a thickened composition containing fumed silica. The method is directed to water and hydrocarbons, and not liquefied gases, and through the inclusion of strongly polar fluids, such as water, is in direct contrast with the findings of the present invention, as this disclosure is directed to the use of non-polar liquefied gases such as LCO2.
Furthermore, the aforementioned documents do not address the need for thickening LCO2 required to ensure optimal fracing with or without a proppant. By adding the fumed silica to the LCO2 with or without additional additives depending on the demands presented by the geological formations, lack of viscosity from chemical thickeners such as gels has been addressed by the present invention physical thickeners.